Apparatus for charging a recording element with an electrostatic charge of a desired amplitude

ABSTRACT

Apparatus for charging a recording element includes a corona generating device that is energized with pulses of a constant amplitude. Circuitry is provided for obtaining an error signal, if the total charge applied to the recording element tends to vary from a preset desired amplitude, and for controlling the width or the frequency of the pulses in response to the error signal to regulate the applied charge. The method of charging the surface of the recording element to a desired amplitude in a given time includes applying an electrostatic charge to the recording element in pulses and adjusting either the width or the frequency of the pulses to obtain the desired amplitude.

nite States atent 1151 3,699,335 Giaimo, Jr. 14 Oct. 17, 1972 [s41 APPARATUS FOR CHARGING A 3,076,092 1/1963 Mott ..2s/49.s RECORDING ELEMENT WITH AN 3,335,274 8/1967 Codichini et al. ..250/49.5 ELECTROSTATIC CHARGE OF A 3,586,908 6/1971 Vosteen ..317/262 X DESIRED AMPLITUDE Primary ExaminerWilliam F. Lindquist [72] Inventor: Edward Charles Gialmo, Jr., Pnn- Attorney G1enn Bruesfle ceton, NJ.

[73] Assignee: RCA Corporation [57] ABSTRACT [22] Filed; Jam 6, 1971 Apparatus for charging a recording element includes a corona generating device that is energized with pulses pp 104,217 of a constant amplitude. Circuitry is provided for obtaining an error signal, if the total charge applied to 52 U.S. c1 .250 4 .sz 1 2 2 the ramming element tends mm Preset {51} int. Cl. G 03 g 1 5/0 desired amplitude and for controlling the width or the [58] Field of Search ..250/49 5 6C 49 5 2c frequency the Pulses respmse the Signal 250/49 1 t e u ate the aae e sharge-r The. m t o c ing the surface of the recording element to a desired amplitude in a given time includes applying an electrostatic charge to the recording element in pulses and [56] References Clted adjusting either the width or the frequency of the pul- UNITED STATES PATENTS ses to obtain the desired amplitude. 2,885,556 5/1959 Gundlach ..250/49.5 6 Claims, 6 Drawing Figures l0? 3 4 my VOL/216E 4 EEZ-i POE Ell? SUP/ U /04 34 V (38 r055 -96 emu/r SW/fC/f w 86 1- 70 PH/ W/flfl/ 9z li 23 78 82 W 470m O/FF 58 P01 m 447/? A 74 F/ifOI/E/VC) 76 t PATENTEBncv n. ma 3.699.335

we VOLT/16E 4% 055 Pom su /2r =/04 1 (38 [62 l 9a SHU/VT P/m /00 I055 -96 emu/r 1 SW/TCH FREOI/f/VC) 76" V 5Z7 INVENTOR.

Edward C. Giaimmlr.

ATTORNEY APPARATUS FOR CHARGING A RECORDING ELEMENT WITH AN ELECTROSTATIC CHARGE OF A DESIRED AMPLITUDE BACKGROUND OF THE INVENTION This invention relates generally to apparatus for and a method of charging a recording element with an electrostatic charge of a'desired amplitude, and, more particularly, to apparatus for and a method of controlling the amplitude of an electrostatic charge applied to the surface of a recording element in a given time.

In an electrophotographic printing system, wherein the surface of a recording element is charged with an electrostatic charge in the course of relative motion between the recording element and a closely spaced corona generating device, the applied electrostatic charge is not always uniformly distributed on the surface. The effect of fluctuations in the charge density is believed to be caused by an overcharging condition, resulting in developed prints of reduced contrast. Overcharged developed direct prints exhibit a reduction in image intensity, and overcharged developed reversal prints exhibit unwanted toning in the background areas.

Since the corona discharge wire of a corona generating device is usually disposed about one-half inch, or less, from the surface of the recording element for effectively charging the recording element, a slow relative motion between a continuously energized corona generating device and the recording element usually causes the recording element to become overcharged. Overcharging cannot be remedied practically by decreasing the voltage that energizes the corona generating device because the minimum voltage necessary to produce a corona discharge is critical, and

should be at least 4,000 volts.

The best developed prints from an electrostatic printing system are not always those which receive a maximum overall electrostatic charge, but rather are those which receive an overall electrostatic charge of am amplitude less than the maximum, that is, an electrostatic charge of a desired amplitude. A less than maximum electrostatic charge may also be desirable to obtain the fastest exposure, greatest contrast, and a minimum of spurious background (noise) toning with certain electr'ophotographic recording elements and electrostatic printing systems.

It has been proposed to control the electrostatic charge on a recording element by charging the recording element with a corona generating device provided with a screen gridi While screen-grid control is suitable for certain electrostatic printing applications, it results in a relatively bulky corona generating device and an inefficient use of the corona discharge.

SUMMARY OF THE INVENTION The novel apparatus and method involves charging a recording element with pulses of corona discharge. Preferably, an error signal is generated when the applied electrostatic charge varies from the desired am-' plitude and is used to control the width or the frequency of the pulses to regulate the applied electrostatic charge.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a fragmentary schematic diagram of one embodiment of an electrostatic printing system for applying a controlled electrostatic charge to the surface of a recording element with the apparatus and method of the present invention;

FIG. 2 is a schematic drawing, partly in block diagram form, of one embodiment of novel apparatus for controlling the frequency or the pulse width of energizing pulses applied to corona generating devices, illustrated in FIG. 1, to regulate the electrostatic charge applied to the recording element;

FIG. 3 is a schematic diagram, partly in block diagram form, of the electrostatic probe and circuitry therefor shown in block diagram in FIG. 2;

FIG. 4 is a schematic diagram of components of the apparatus illustrated in block diagram form in FIG. 2;

FIG. 5 is a series of wave forms used to explain the operation of the novel apparatus illustrated in FIGS. 2, 3, and 4; and

FIG. 6'is a graph of the surface potential acquired by three different recording elements resulting from charging the surface of these recording elements with pulses of different pulse widths.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1, there is shown an electrostatic printing system 10 of the type wherein an electrophotographic recording element 12 is moved along a path indicated by arrows adjacent to the recording element 12. The recording element 12 comprises a photoconductive layer 14 on a relatively conductive backing 16. The recording element 12 is moved along its path, at a substantially constant speed, by any suitable means (not shown), past a charging station 18, an exposure station 20, and a developing station 22, as for example described in detail in U.S. Pat. No. 3,517,994,

' issued to M. A. Leedom on June 30, 1970 for Electrophotographic Apparatus. The recording element 12 is charged at the charging station 18 by corona generating devices 24 and 26 disposed on opposite sides of the recording element 12, each at a distance of about one-half inch from the recording element 12. The corona generating devices 24 and 26 are of the type described in U.S. Pat. No. 2,922,883, issued to E. C. Giaimo, Jr. on Jan. 26, 1960. The electrostatically charged recording element 12 is exposed at the exposure station 20 by light images on a thin-window cathode-ray tube 28 to produce electrostatic latent images which are developed at the developing station 22 by liquid developer 30 from a developing tank 32.

The corona generating devices 24 and 26 are energized with pulses of opposite polarity, respectively, in a manner to be hereinafter explained. The recording element 12 would normally be overcharged if the corona generating devices 24 and 26 were energized continuously, especially when the relative motion between the corona generating devices 24 and 26 and the recording element 12 is relatively slow. In the apparatus 10, the speed of movement of the recording element 12 is generally less than I inch/second.

Means are provided to sense the electrostatic charge applied to the recording element 12 by the corona generating'devices 24 and 26. To this end, an electrostatic probe 34 is disposed adjacent the photoconductive layer 14 of the recording element 12 at an electrostatic voltage sampling station 36, between the charging station 18 and the exposure station 20. The probe 34 is disposed about one-eighth inch from the photoconductive layer 14.

The probe 34 and its associated probe circuit 38, as shown in FIG. 3, is an electrostatic voltmeter of the type manufactured by Monroe Electronics'Ine, Middleport, New York, 14105, under the trademark .ISOPROBE, Model No. 144 S-4 with Detector/Preamplifier Probe Model 10098. An explanation of the operation of the probe 34 and its probe circuit 38, substantially as described in the instruction book for this equipment, is as follows: 1 An electrostatic electrode 40 (FIG. 3) looks at (senses) the charged surface of the recording element 12 through a small hole 42 at the base 44 of the probe 34. A metal vane 46 is vibrated by an electromechanical vane driver 48 that isenergized by' an oscillator circuit 50. The chopped ac signal thus induced on the electrode 40 is proportional to the differential voltage between the surface (of the recording element 12-) under measurement and the probe 34. The phase of the chopped ac signal is'dictated by the dc polarity.

A reference voltage derived from the oscillator circuit 50 and the mechanically modulated signal, conditioned by a high input impedance preamplifier 52 and signal amplifier 54, are fed through isolation transformers 56 and 57 to a phase sensitive detector 58 whose output dc amplitude and polarity are dictated by the amplitude and phase of the electrostatically induced signal relative to the reference voltage. The output of the phase sensitive detector 58 feeds a high level dc integrating amplifier 60. The output of the integrating amplifier 60; whose polarity is identical 'to the unknown, is fed directly to the probe 34. As the open loop gain from the .probe 34 to the integrating amplifier 60 is extremely high (approximately the probe 34 is One contact 72 of the switch 70 is applied to one input of a differential amplifier 74. A variable reference voltage 76 is applied to another input of the differential amplifier 74 which amplifies the difference of the input voltages.

The output of the differential amplifier 74 is applied, through a double-pole, single-throw switch 78, to a motor 800 (FIG. 4) of a motor-operated double (ganged) potentiometer 80. The motor-operated potentiometer 80 is of the type sold by Automation Development Company, Culver City, California, or by Industrial Control Company, Farmingdale, New York, for example The motor-operated potentiometer 80 is part of a free-running multivibrator circuit 82, shown in detail in'FIG, 4. The motor 80a of the motor-operated potentiometer 80 is a reversible motor adapted to increase ordecrease the resistance of the potentiometer 80 in response to the polarity and amplitude of the (error) signal received from the differential amplifier 74, whereby to decrease or increase the frequency of an output square wave from the multivibrator circuit 82, in a manner known in the art. The square-wave output of the multivibrator circuit 82 is applied to the input of a monostable flip-flop circuit 84.

The flip-flop circuit 84, shownin detail in FIG. 4, is adapted to be triggered by the leading edge of the square-wave outputs from the multivibrator circuit 82. A variable resistor 86 in the flip-flop circuit 84 (FIG. 4) is varied by a motor 86a to vary the pulse widths of the pulses produced by the flip-flop circuit 84 in a manner to be hereinafter explained.

Another contact 88 of the switch 70 is connected to one input of a differential amplifier 90. A variable reference voltage 92 is connected to another input of the differential amplifier 90. The output, the difference of the input voltages, of the differential amplifier 90 is driven to a dc voltage typically within 0.01 percent of the potential of the unknown for one-eighth inch probe-to-surface spacing. By simply metering the output of the dc integrated amplifier 60, as with a voltmeter 62, an accurate indication of the unknown potential (electrostatic charge) is provided. The output voltage from the amplifier 60 is used to obtain an error signal in a manner to be described hereinafter in detail. This output voltage from the amplifier 60 may also be utilized simultaneously with the voltmeter 62 to provide I charging characteristics of recording elements, as will be hereinafter described.

Referring now to FIG. 2, the novel apparatus for charging the recording element 12 with an electrostatic charge of a desired amplitude is shown. Electrostatic charges of oppositepolarity are applied to the opposite surfaces, respectively, of the recording element 12 by pulses applied to the corona generating devices 24and 26 ha manner to be described hereinafter. The electrostatic charge on the surface of the photoconductive layer 14 is detected by the probe 34 at the voltage (measuring) sampling station 36 and applied to the probe circuit 38, as described in FIG. 3. The output of the probe circuit 38, that is, the amplified detected voltage at the measuring station 36, is applied to an arapplied to the input of the motor 86a through a doublepole, single-throw switch 94 to vary the resistance of the resistor 86 in the flip-flop circuit 84. The output of the flip-flop circuit 84 is applied to a shunt-tube switch circuit 96, shown in detail in FIG. 4.

The shunt-tube switch circuit 96 is connected between a high-voltage power supply 98 and the corona generating device 24 to apply energy to the corona generating device 24 in pulses, causing the devices .24 and 26, to generate corona discharges of opposite polarity in pulses, in a manner hereinafter to be explained. The positive terminal of the power supply 98 is connected to the shunt-tube switch circuit 96 through a load resistor 100. The resistor of a potentiometer 102 is connected between the positive and negative terminals of the power supply 98 and the midpoint of the resistor is grounded by a rotor 104.

The power supply 98 provides at least 6,000 volts so that when a voltage is applied between the corona generating devices 24 and 26, the voltage applied to the corona generating device 24 is about 3,000 volts above ground and the voltage applied to the corona generating device 26 is about 3,000 volts negative with respect to ground. Under these conditions, the corona generating devices 24 and 26 generate corona discharges which charge the surface of the photoconductive layer 14 positively and the surface of the backing l6 negatively. In operation, the dc voltage applied to one corona generating device is insufficient to provide a corona discharge. By applying a high voltage pulse to the other corona generating device, the voltage between both corona generating devices is sufficient to produce corona discharges from both devices.

The multivibrator circuit 82, shown in detail in FIG. 4, is a free-running square-wave oscillator of a type well known in the art. Briefly, the multivibrator circuit 82 comprises two triode tubes 104 and 106 wherein the anode of the tube 104 is coupled to the grid of the tube 106 through one of a plurality of capacitors, as determined by a switch 108. Similarly, the anode of the tube 106 is coupled to the grid of the tube 104 through one of a plurality of capacitors, as determined by a switch 110. The switch 110 is ganged to the switch 108, as indicated by the dashed line 112, for contacting capacitors of 'different capacitance. The frequency of the free-running multivibrator circuit 82 can be selected, that is, preset, by positioning the switches 108 and 110 to selected contacts, and the selected frequency may be varied over a range of frequencies if the reversible motor 80a, coupled to the ganged potentiometer 80, varies the resistance in the multivibrator circuit 82. The values of the resistors in FIG. 4 are in ohms, and the values of the capacitors are in microfarads, unless otherwise indicated. Thus, the multivibrator circuit 82 is capable of providing pulses at a frequency in the range between 0.2 to 20,000 pulses per second.

The flip-flop circuit 84 is monostable in operation and will provide a square-wave output every time it is triggered by the leading edge of a square-wave pulse from the multivibrator circuit 82. The output of the multivibrator circuit 82, at the anode of the tube 106, is coupled-to the input of the flip-flop circuit 84 through a coupling capacitor 114. In the flip-flop circuit 84, the anode of a triode tube 116 is coupled to the grid of a triode tube 118 through a capacitor 120. The grid of the tube 118 is connected to ground through the variable resistor 86 and resistors 120 and 122. The anode of the tube 118 is connected to the grid of the tube 116 through a resistorcapacitor network 124. By varying the resistance of the variable resistor 86, as by the motor 860 coupled to the variable resistor 86, the pulse widths of the pulses produced by the monostable flipflop circuit 84 are varied. The pulse width of a square wave pulse may be varied in the range between 0.010 and 4.0 seconds, using the values of the components indicated in FIG. 4. The output from the flip-flop circuit 84 is taken from a potentiometer 126 whose resistor is connected between the anode of the tube 118 and ground.

The shunt-tube switch circuit 96 (FIG. 4) comprises a triode 128 whose cathode is connected to ground, through a gas tube 130, and whose anode is connected to the positive terminal of the high voltage supply 98, through the load resistor 100, as explained for the circuit in FIG. 2. The corona generating device 24 is connected directly to the anode of the tube 128, and the corona generating device 26 is connected to the negative terminal of the power supply 98, as explained previously. The output of the flip-flop circuit 84 is connected to the grid of the tube 128 through a rotor 134 of the potentiometer 126.

The electrostatic charging of the recording element '12 with pulses will now be described with reference to the circuit diagram of FIG. 4 and the pulse diagrams of FIG. 5. Let it be assumed that multivibrator circuit 82 is providing square-wave output pulses 140 at the anode of the tube 106 at a frequency f1, as shown in FIG. 5. The duration of one cycle of the output of the multivibrator circuit 82 is Tl plus T2. The flip-flop circuit 84 is triggered on by the leading edge of every positive input pulse 140 and provides pulses 142 at its output, the rotor 134 of the potentiometer 126.

The tube 128 of the shunt-tube switch circuit 96 is biased so that each of the pulses 142 triggers the tube 128 into conduction which, in turn, essentially short circuits one-half of the power supply 98 so that the energy provided by the power supply 98 to the corona generating devices 24 and 26 are not sufficient to produce a corona discharge. Thus, when the tube 128 is conducting for a period of time T3 (FIG. 5), the corona generating devices 24 and 26 are not energized. When the shunt tube 128 is not conducting, as during the time period T4, energy is supplied to the corona generating devices 24 and 26 and they produce positive and negative coronas, respectively.

If the recording element 12 is provided with an excessive amount of electrostatic charge, as when the time period T4 is too long, the frequency of the multivibrator circuit 82 can be increased either by varying the capacitance of the circuit through the switches 108 and 110 or by decreasing the resistance of the variable resistors 80. Pulses 144 (FIG. 5) represent pulses of the same amplitude as the pulses but they are produced at a higher frequency, 12. Under the conditions of frequency f2, the shunt tube 128 is on for a period T7 and off for a period T8. The flip-flop circuit 84 under these latter conditions is switched on at a higher rate than it was previously, that is, it is switched on for a period of T7 and switched off for a period T8, as indicated by the pulses 146 in FIG. 5. Since the period T8 is smaller than the period T4, the shunt tube 128 will be off for less time than previously stated and the power supply 98 will energize the corona generating devices 24 and 26 for a lesser period of time than previously.

If it's desired to energize the corona generating devices 24 and 26 even less than previously, the pulse width of the pulses produced by the flip-flop circuit 84 can be increased by increasing the resistance of the variable resistor 86. Thus, pulses 148 having a duration of T9 can be produced, using the pulses 144 of the multivibrator circuit 82 to trigger the flip-flop circuit 84. Since the pulses of the flip-flop circuit 84 are triggered by the leading edge of the pulses from the multivibrator circuit 82, the pulses 148 of the flip-flop circuit 84 are triggered by every third pulse 144 of the multivibrator circuit 82, the latter running at a frequency of f2. This occurs because the time T9 is greater than the sum of the times T5 and T6. Under the latter conditions, the shunt tube 128 will be triggered on by the long pulse 148 for the relatively long time period T9 and would be triggered off for the relatively shorter time period T10. During the off" time T10, the corona generating devices 24 and 26 are energized by the power supply 98.

Referring now to FIG. 2, the operation of the novel apparatus for charging the recording element 12 with a regulated electrostatic charge of a desired amplitude will be explained: Let it be assumed that the recording element 12 is being moved, in the direction of the arrows associated therewith, at a constant speed past, and between, the corona generating devices 24 and 26. Let it also be assumed that the settings of the switches 108, 110 and the resistors 80 of the multivibrator 82 are set to provide a desired charge on the recording element 12. Let it be assumed further that the variable resistor 86 is set to provide output pulses from the flip-flop circuit 84 of a desired pulse width so that the corona generating devices 24 and 26 are energized periodically by the power supply 98 to provide the desired electrostatic charge on the recording element 12.

The probe 34 senses the electrostatic charge that passes by it, at the voltage sampling station 36, and applies it to the probe circuit 38. The output of the probe circuit 38, which is indicated on the voltmeter 62, is applied to the armature 68 of the switch 70. The armature 76,- no error signal is applied to the motor 80a, and the rotor of the potentiometer 80-remains stationary. If,

' however, the voltage applied to the recording element 12 tends to increase, this increase in voltage is sensed by the probe 34 and the output from the probe circuit 38 to the differential amplifier 34 is increased. This increased voltage is compared'with the preset reference voltage 76 and produces an error signal at the output of the differential amplifier 74 to rotate the reversible motor 80a and to move the rotor of the potentiometer 80 in a direction to increase the output frequency of the multivibrator circuit 82. Under these latter conditions, the shunt tube 128 is triggered off for a'lesser duration'of time, (i.e., T8 now as against T4 previously), and the corona generating devices 24 and 26 are energized by pulses less frequently than previously. Hence, any tendency for the voltage on the recording element 12 to increase is automatically regulated by providing fewer pulses per unit of time to the corona generating devices 24 and 26, thereby regulating the electrostatic chargeapplied to the recording element 12. Conversely, any tendency for the voltage on the recording element 12 to decrease results in an error signal of a polarity which decreases the frequency of the multivibrator circuit 82, increases the of time of the flip-flop circuit 84, and energizes the corona generating devices 24 and 26 longer per unit of time.

The voltage (electrostatic charge) applied to the recording element 12 may also be regulated by varying the pulse width of the pulses provided by the flip-flop circuit 84. In this mode of operation, the armature 68 of the switch 70 is moved to make electrical contact with the contact 88 (FIG. 2). The switch 78 is opened, the switch 94 is closed, and the frequency of the multivibrator circuit 82 is preset to a fixed frequency. Any variation in the electrostatic voltage applied to the recording element 12 is sensed by the probe'34, applied to the probe circuit 38, and the output of the probe circuit 38 is compared with the preset reference voltage 92 by the differential amplifier 90. Error signals at the output of the differential amplifier 90 are applied to the motor 80a through the now closed switch 94 to vary the resistance of the resistor 86 degeneratively. Thus, the resistance of the resistor 86 is either increased or decreased depending upon whether less or more electrostatic charging respectively, of the recording element 12 is needed to regulate the electrostatic charge.

- Instead of using two double corona generating devices 24 and 26, a single corona generating device 24 may be used and the corona generating device 26 can be replaced with a metal plate adjacent, and in contact with, the surface of the backing 16 of the recording element 12. Under these latter conditions, the metal plate would be connected to one terminal of the power supply 98 and grounded, and the corona generating device 24 would be connected to the other terminal of the power supply 98;

The novel method of charging the recording element with an electrostatic charge of a desired amplitude in a given time will be described with reference to the graphs in FIG. 6. Referring now to FIG. 6, there are shown three graphs A, B, and C which represent the relationships-between potentials on the surfaces of I three different recording elements A, B, and C resulting from charging pulses of different pulse widths.

Data for the Graphs A, B, and C is of the type obtained by using the apparatus shown in FIG. 1. A recording element A of one manufacturer was moved past the corona generating devices 24 and 26 at a speed of 0.3 inches/second. Corona generating devices 24 and 26 were energized with substantially constant amplitude pulses of opposite polarity, respectively, from a power supply to provide corona discharges. The Graph A shows the relationship between the charge on the recording element A, as measured by the probe 34 and read on the output voltmeter 62 (FIGS. 2 and 3), and the pulse widths of the pulses applied to the corona generating devices 24 and 26. The curves B and C were obtained in a manner similar to that of the curve A except that recording elements B and C of different manufacturers were used.

It has been observed that in reproducing images by electrophotographic apparatus, such as the apparatus 10 in FIG. 1 in which reversal development and a development electrode was used, the best reproduction was not always obtained when the recording element was charged to a maximum. In fact, the same amount of electrostatic charge for the different recording elements A, B, and C did not produce the same results. For example, a surface potential of 600 volts on the recording element C may give the same results as a surface charge of 340 volts on the recording element B or 250 volts on the recording element A. Once a determination of the relationship between the potential on the surface of a particular recording element and the pulse width of pulses for energizing the corona generating devices is made, a selection of the desired electrostatic charge for optimum results on the moving recording element can be made by adjusting the pulse width of the pulses applied to the corona generating devices 24 and 26. This is accomplished by adjusting the reference voltage 92 while watching the voltmeter 62. It is also possible, as described previously, to adjust corona generating device and said surface is subthe total electrostatic charge on the moving recording stantially constant, element by varying the frequency of the pulses instead said energizing means includes a power supply of their pulse widths. This is accomplished by varying adapted to provide an output voltage that is subthe reference voltage 76 while watching the voltmeter n i lly C St t and at least Sufficient to 62 until the desired voltage for optimum results is inproduce a corona discharge from a c o a di m d, discharge wire, and

If the electrostatic printing system is one wherein the Said energizing means comprises Switching means to electrostatic charge on the recording element is not connect and disconnect said Power pp y to Said regulated, the novel method may still be practiced if discharge wlrein p to Said error means are provided to vary either the frequency or the 518m], whereby to Prowde Sald Voltage Pulses of a pulse width of the pulses applied to the corona generatconstant ampmufie- .ing devices. For example, if the charging apparatus of Flectfostanc 'f system of the type the printing system comprised only the circuitry indescflbedm clam a'fvherem dicated inFIG 4, without the motors 80a and 86a, sal d means to obtain an error slgnalcomprlses an adeither the frequency or the pulse-width of the charging Justable voltage a wherfaby to pulses could be controlled manually by either varying select or preset sald desired amplltude of said electhe resistance of the resistors 80 in the multivibrator trostatlc charge 1 h l circuit 82 or the resistor 86 in the flip-flop circuit 84, probef meansdto Samp e t 6 v0 tage app led to Sal respectively. Sur an means to compare said reference voltage with the lclaiml sampled voltage, whereby to obtain said error 1. In an electrostatic printing system of the type SignaL wherein acorona generating device is disposed to apply 5 A electrostatic imi System f the type an electrostatic charge to a surface, the improvement d ib i l i 4 h i of means to regulate the electrostatic charge resulting id f db k means comprises on Said Surface at a desired amplitude p g: a free-running multivibrator circuit having means to electrical energizing means to energize said corona vary its frequency in response to said error signal,

generating device with voltage pulses of a constant and amplitude, whereby to apply an electrostatic means to apply the output of said multivibrator circharge to said surface, cuit to said switching means to control the means to obtain an error signal when the amplitude frequency at which said power supply is connected of the electrostatic charge on said surface varies and disconnected to said corona discharge wire. from said desired amplitude, and 6. An electrostatic printing system of the type feed-back means, responsive to said error signal, to described in claim 4, wherein control the width or the frequency of said voltage said feed-back means comprises pulses of a constant amplitude, wher b t r a free-running multivibrator circuit having means to late the electrostatic charge on said surface. set its frequency of output at a constant frequency, 2. An electrostatic printing system of the type a monostable pp circuit having an p ib d i l i 1 h i responsive to the output of said multivibrator cirsaid corona generating device comprises at least one cult, i having an output connected to Said corona discharge wire disposed for relative motion Swltchmg means" and with respect to, and adjacent, said surface, and means to p y 531d lgnal to 531d P- p f' said surface is a surface of a recording element. clllt to y h P1115e wldth Of the output of Sam 3. An electrostatic printing system of the type P' P clrculti wllereiby to control the rate of described in claim 2 wherein switching of said switching means. the speed of said relative motion between said 

1. In an electrostatic printing system of the type wherein a corona generating device is disposed to apply an electrostatic charge to a surface, the improvement of means to regulate the electrostatic charge resulting on said surface at a desired amplitude comprising: electrical energizing means to energize said corona generating device with voltage pulses of a constant amplitude, whereby to apply an electrostatic charge to said surface, means to obtain an error signal when the amplitude of the electrostatic charge on said surface varies from said desired amplitude, and feed-back means, responsive to said error signal, to control the width or the frequency of said voltage pulses of a constant amplitude, whereby to regulate the electrostatic charge on said surface.
 2. An electrostatic printing system of the type described in claim 1, wherein said corona generating device comprises at least one corona discharge wire disposed for relative motion with respect to, and adjacent, said surface, and said surface is a surface of a recording element.
 3. An electrostatic printing system of the type described in claim 2, wherein the speed of said relative motion between said corona generating device and said surface is substantially constant, said energizing means includes a power supply adapted to provide an output voltage that is substantially constant and at least sufficient to produce a corona discharge from said corona discharge wire, and said energizing means comprises switching means to connect and disconnect said power supply to said corona discharge wire in response to said error signal, whereby to provide said voltage pulses of a constant amplitude.
 4. An electrostatic printing system of the type described in claim 3, wherein said means to obtain an error signal comprises an adjustable reference voltage means, whereby to select or preset said desired amplitude of said electrostatic charge, probe means to sample the voltage applied to said surface, and means to compare said reference voltage with the sampled voltage, whereby to obtain said error signal.
 5. An electrostatic printing system of the type described in claim 4, wherein said feed-back means comprises a free-running multivibrator circuit having means to vary its frequency in response to said error signal, and means to apply the output of said multivibrator circuit to said switching means to control the frequency at which said power supply is connected and disconnected to said corona discharge wire.
 6. An electrostaTic printing system of the type described in claim 4, wherein said feed-back means comprises a free-running multivibrator circuit having means to set its frequency of output at a constant frequency, a monostable flip-flop circuit having an input, responsive to the output of said multivibrator circuit, and having an output connected to said switching means, and means to apply said error signal to said flip-flop circuit to vary the pulse width of the output of said flip-flop circuit, whereby to control the rate of switching of said switching means. 